1. Field of the Invention
The prevent invention relates to a rotary connector configured so as to enable extension or contraction of a cable transmitting electrical signals etc. by connecting a spirally wound flat cable between two cable parts and making the spirally wound flat cable wind or unwind using a rotary body and a fixed body, more particularly relates to a rotary connector able to protect the flat cable from fusing when an overcurrent flows.
2. Description of the Related Art
When transmitting electrical signals over a cable, it is sometimes desired to increase or decrease the length of the cable. In addition, when attaching an electrical cable to the steering column of a car etc., it is necessary to prevent the cable from being stretched or sagging due to the rotation of the steering column.
In these cases, the objective can be achieved by giving the cable a sufficient extra length and allowing the cable to flex by that extra length, but there are limits to the extra length. If repeatedly flexed, further, stress fatigue will occur in the cable and it may break. Further, if the cable is allowed to sag by the excess length, it will cause difficulties in the arrangement of other parts and will detract from the appearance. Further, when laying such a cable in a car for the transmission of electrical signals, in many cases conditions do not allow excess lengths of cable to be laid or cable to flex by the excess length.
A rotary connector is used to overcome this disadvantage. An example of such a rotary connector is shown in FIG. 1 and FIG. 2. FIG. 1 is a perspective view of the appearance of a rotary connector, while FIG. 2 is a view of the parts of the rotary connector shown in FIG. 1 in a disassembled state.
The rotary connector illustrated is comprised of an inside case 1 and an outside case 2. The inside case 1 and outside case 2 are combined so as to be able to rotate relative with each other. The outside case 2 is integrally comprised of an upper cover 2a, a lower cover 2b, and an inlet portion 2c accommodating a connection portion. A spirally wound flat conductive cable, that is, a flat cable 4, is accommodated in a space 3 formed by the inside case 1 and outside case 2 when the outside case 2 and the inside case 1 are combined to be able to rotate relative to each other. In the illustrated example, the flat cable 4 is wound clockwise from the outside to the inside.
Cables, that is, outer conductors 5A and 5B (here referred to together as the outer conductors 5), are connected to the conductor portions at the two ends of the flat cable 4. In other words, the outer conductors 5A and 5B are connected through the flat cable 4. The connection portions 6A and 6B between the conductor portions at the two ends of the flat cable 4 and the outer conductors 5A and 5B are protected by molded pieces of plastic. The connection portion 6A is accommodated and affixed in the connection accommodating portion 1a of the inside case 1, while the connection portion 6B is accommodated and affixed in the connection accommodating portion 2c of the outside case 2.
A rotary connector structured in the above way can wind up the flat cable 4 spirally wound clockwise from the outside to the inside in the space 3 by the inside case 1 turning clockwise with respect to the outside case 2. Conversely, it can unwind the flat cable 4 spirally wound in the space 3 by the inside case 1 turning counterclockwise with respect to the outside case 2. When the flat cable 4 is wound in the counterclockwise direction from the outside to the inside, the relationship between the direction of rotation of the inside case 1 and the winding and unwinding of the flat cable 4 becomes opposite to the above.
Next, an example of the use of the above rotary connector will be explained. When connecting an electrical circuit the steering column of a car and supplying power or transmitting control signals to the electrical circuit through the outer conductors 5A and 5B from the outside, if the inside case 1 is affixed to the steering column and the outside case 2 is affixed near the steering column, the inside case 1 turns in accordance with the rotation of the steering column and the flat cable 4 is unwound or wound up. Since the distance between the ends of the outer conductors 5A and 5B does not change, however, the outer conductors 5A and 5B are not stretched or do not sag around the steering column. As a result, it is possible to supply power or transmit control signals from the outer conductors 5A and 5B without being limited by the rotation of the steering column.
FIG. 3 is a view showing an example of the configuration of the electrical circuit using a rotary connector of the above structure. This electrical circuit is comprised of a power supply side circuit provided with a battery 7 and a fuse box 8 connected at an external conductor 5B side connected to the flat cable 4 and a load 9 connected to the other external conductor 5A side connected to the flat cable 4. Reference numerals 5a are connection portions of the ends of the external conductors 5A and 5B and a wire harness. Reference numeral 6+ shows the plus (+) side connection portion connected to the positive (+) side of the power supplied from the battery 7, or the power supply side, among the connection portions 6A and 6B, while reference numeral 6 - shows the negative (-) side connection portion connected to the negative (-) side.
In the above mentioned rotary connector, however, when a short-circuit occurs somewhere in a circuit including the flat cable 4 and the outer conductors 5A and 5B shown in FIG. 3, specifically, when a short-circuit occurs at a circuit of the load 9 side, the problem is encountered of insufficient protection of the flat cable 4.
Explaining this in more detail, as shown in FIG. 3, a fuse box 8 for protection against overcurrent is connected to the positive terminal (+) side (power supply side) of the battery 7. When a short-circuit occurs in the circuit, the fuse provided in the fuse box 8 fuses due to the overcurrent and protects the circuit. The fuse provided in the fuse box 8 however is a large capacity one of about 10 to 15A (amperes) due to the relationship with the other loads etc. not shown. Since the flat cable 4 provided between the outer conductor 5A and the outer conductor 5B must have pliability, the cross-sectional area of the flat cable 4 is usually smaller than the cross-sectional area of the outer conductors 5A and 5B and so the current capacity of the flat cable 4 is smaller than the current capacity of the outer conductors 5A and 5B. That is, the fusing current of the flat current is naturally lower than the fusing current of the outer conductors 5A and 5B. Further, the value of the fusing current of the fuse of the fuse box 8 is normally designed in consideration of the outer conductors 5A and 5B, the load, etc. and is not designed in consideration of the current capacity of the flat cable 4. As a result, the fusing current of the flat cable 4 is often lower than the fusing current of the fuse. For example, the current capacity of the flat cable 4 is about 5 amperes. Therefore, with the above method of protection, the problem is encountered that when a short-circuit etc. occurs and an overcurrent flows to the electrical circuit, the flat cable 4 will heat and end up fusing due to the overcurrent before the fuse in the fuse box 8 fuses. That is, with the above method of protection, the protection of the flat cable 4 is insufficient.
Note that in the above rotary connector, an example of use of a flat cable 4 was shown from the viewpoint of the ease of winding and unwinding. As explained above, however, even when using a cable with a usual round cross-section able to wind and unwind, a similar problem is encountered. The flat cable 4 or the cable accommodated inside the rotary connector corresponding to the same is called an "inner conductor" below, while the cables corresponding to the outer conductors 5A and 5B positioned outside the rotary connector are called "outer conductors".